U.S. patent application number 12/366336 was filed with the patent office on 2009-06-04 for linear voltage controlled variable attenuator with linear db/v gain slope.
This patent application is currently assigned to Freescale Semiconductor, Inc.. Invention is credited to Lawrence E. Connell, Neal W. Hollenbeck, Daniel P. McCarthy.
Application Number | 20090143036 12/366336 |
Document ID | / |
Family ID | 37894743 |
Filed Date | 2009-06-04 |
United States Patent
Application |
20090143036 |
Kind Code |
A1 |
McCarthy; Daniel P. ; et
al. |
June 4, 2009 |
LINEAR VOLTAGE CONTROLLED VARIABLE ATTENUATOR WITH LINEAR DB/V GAIN
SLOPE
Abstract
A variable attenuator and method of attenuating a signal is
presented. The variable attenuator contains an input that receives
an input signal to be attenuated. A voltage divider between a
resistor and parallel MOSFETs provides the attenuated input signal.
The MOSFETs have different sizes and have gates that are connected
to a control signal through different resistances such that the
larger the MOSFET, the larger the resistance. The control signal is
dependent on the output of the attenuator. The arrangement extends
the linearity of the attenuation over a wide voltage range of the
control signal and decreases the intermodulation distortion of the
attenuator.
Inventors: |
McCarthy; Daniel P.; (Elk
Grove Village, IL) ; Connell; Lawrence E.;
(Naperville, IL) ; Hollenbeck; Neal W.; (Palatine,
IL) |
Correspondence
Address: |
BRINKS, HOFER, GILSON & LIONE;FREESCALE
P.O. BOX 10395
CHICAGO
IL
60610
US
|
Assignee: |
Freescale Semiconductor,
Inc.
|
Family ID: |
37894743 |
Appl. No.: |
12/366336 |
Filed: |
February 5, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11238657 |
Sep 28, 2005 |
7505748 |
|
|
12366336 |
|
|
|
|
Current U.S.
Class: |
455/249.1 |
Current CPC
Class: |
H03G 3/3063 20130101;
H03G 1/007 20130101 |
Class at
Publication: |
455/249.1 |
International
Class: |
H04N 7/00 20060101
H04N007/00 |
Claims
1. A tuner comprising: a mixer configured to down-convert input
signals; a variable attenuator configured to attenuate the
down-converted input signals, the variable attenuator containing a
plurality of parallel elements and a first voltage divider that is
connected to a control terminal, the parallel elements connected to
different nodes of the first voltage divider; and a first amplifier
configured to amplify the attenuated signals, attenuation of the
variable attenuator controlled using feedback from the first
amplifier.
2. The tuner of claim 1, wherein the variable attenuator further
comprises a second voltage divider containing a series circuit that
includes a resistor and the parallel elements, a node between the
resistor and the parallel elements connected to an output of the
variable attenuator.
3. The tuner of claim 2, wherein the parallel elements vary over
different ranges of resistance.
4. The tuner of claim 3, wherein a parallel element that varies
over a smaller range of resistance is connected to the control
terminal through a smaller resistance than a parallel element that
varies over a larger range of resistance.
5. The tuner of claim 1, further comprising: an automatic gain
controller configured to receive the feedback from the first
amplifier and provide a control signal to the control terminal of
the variable attenuator.
6. The tuner of claim 5, wherein the automatic gain controller is
further configured to: set the control signal to increase the
attenuation of the variable attenuator when the feedback is within
a first predetermined voltage from a maximum output voltage; and
set the control signal to decrease the attenuation of the variable
attenuator when the feedback is within a second predetermined
voltage from a minimum output voltage.
7. The tuner of claim 5, wherein the control signal comprises a
discrete digital voltage.
8. The tuner of claim 1, further comprising: a low noise amplifier
disposed between an input of the tuner and the mixer; and a second
amplifier disposed between the mixer and the variable
attenuator.
9. An electronic device comprising the tuner of claim 1.
10. The electronic device of claim 9, wherein the electronic device
comprises a television.
11. The electronic device of claim 9, wherein the input signals
comprise television signals.
12. The electronic device of claim 9, wherein the electronic device
is a mobile electronic device.
13. The tuner of claim 1, wherein the tuner is a television
tuner.
14. A method of tuning, comprising: down-converting an input
signal; providing the down-converted signal to a circuit comprising
a first element and a plurality of parallel elements; providing a
control signal to a voltage divider, the parallel elements
connected to different nodes of the voltage divider, the control
signal controlling attenuation of the down-converted signal by
controlling resistances of the parallel elements; providing an
attenuated signal based on the down-converted signal; and
amplifying the attenuated signal to generate an amplified
attenuated output signal, wherein attenuation of the down-converted
signal is dependent upon the amplified attenuated output
signal.
15. The method of claim 14, wherein the parallel elements vary over
different ranges of resistance.
16. The method of claim 15, wherein a parallel element that varies
over a smaller range of resistance is connected to the control
terminal through a smaller resistance than a parallel element that
varies over a larger range of resistance.
17. The method of claim 14, further comprising: amplifying the
down-converted signal prior to providing the down-converted
television signal to the circuit; and amplifying the input signal
prior to down-converting.
18. The method of claim 14, wherein providing the control signal
comprises: setting the control signal to increase attenuation of
the down-converted signal when the amplified attenuated output
signal is within a first predetermined voltage from a maximum
output voltage; and setting the control signal to decrease
attenuation of the down-converted signal when the amplified
attenuated output signal is within a second predetermined voltage
from a minimum output voltage.
19. The method of claim 18, wherein the input signal is a
television signal.
20. A method of tuning in accordance with claim 14 wherein the
control signal comprises a discrete digital voltage.
Description
PRIORITY CLAIM
[0001] This application is a divisional of co-pending U.S. patent
application Ser. No. 11/238,657, filed Sep. 28, 2005, which is
incorporated by reference.
TECHNICAL FIELD
[0002] The present application relates to a variable attenuator.
More specifically, the present application relates to a voltage
controlled variable attenuator having a linear gain slope.
BACKGROUND
[0003] Televisions contain a number of components including a
screen such as a cathode ray tube (CRT), liquid crystal display
(LCD) or plasma display, as well as circuitry to receive a signal
to be displayed and display the signal on the screen. This
circuitry includes a tuner that tunes to a particular carrier
(frequency) to receive the desired signal.
[0004] Television tuners receive carriers over a predetermined
range, e.g. from 50 MHz to 860 MHz over some standards. This is a
relatively wide range of frequencies compared to other electronic
devices, such as cellular telephones, which operate over a
frequency range of 100 MHz or so. The incoming signal power to a
tuner can vary by many orders of magnitude depending on a variety
of factors such as distance of the tuner to the signal source or
the environmental conditions, for example. Moreover, the signal
strength may vary continuously if the television is mobile as the
television is being transported.
[0005] However, the change in signal strength is problematic as
before the signal is provided to an output of the tuner, it is
amplified by an amplifier having a constant gain. In this case, the
signal strength provided to the amplifier is adjusted so that the
signal is within the range of the amplifier. That is, if the
amplitude of the signal is too large, the amplifier will distort
the peaks of the signal and thus degrade the output of the tuner.
Accordingly, there exists a need to continuously control the gain
of the tuner such that a constant output power level is
achieved.
[0006] To control the gain of the tuner, a variable attenuator is
usually provided between the input signal and the amplifier. It is
desirable to have a continuous gain control using an analog control
voltage. Likewise, it is desirable to have the slope of the gain
linear in dB/V. However, the gain variability function adds
complexity, noise, and distortion problems. For example, a tradeoff
exists between the sensitivity of the attenuation control voltage
and the attenuation range. For increasing attenuation ranges, it
becomes more difficult to maintain a linear in dB/V gain slope.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a block diagram of circuitry in a tuner in
accordance with an embodiment of the invention.
[0008] FIG. 2 illustrates an embodiment of the variable attenuator
in accordance with an embodiment of the invention.
[0009] FIG. 3 is an enlargement of input section 210 of the tuner
in FIG. 2.
[0010] FIG. 4 is a plot of attenuation verses control voltage for a
single MOSFET variable attenuator and three MOSFET variable
attenuator in accordance with an embodiment of the invention.
[0011] FIG. 5 illustrates an embodiment of the variable attenuator
in accordance with an embodiment of the invention.
[0012] FIG. 6 illustrates an embodiment of the variable attenuator
in accordance with an embodiment of the invention.
[0013] FIG. 7 illustrates an embodiment of the variable attenuator
in accordance with an embodiment of the invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0014] A variable attenuator and tuner are provided in which the
variable attenuator is highly linear, has a large automatic gain
control range, and is easily implemented. In addition, the variable
attenuator provides a constant attenuation slope and an improvement
in intermodulation distortion. This enables integration of an
automatic gain control amplifier into the tuner, which reduces cost
and increases flexibility in design of the tuner.
[0015] FIG. 1 illustrates a block diagram of circuitry in a tuner
in accordance with an embodiment. Although other circuitry may be
present, such as an analog-to-digital (A/D) converter or coupling
capacitors, such circuitry is not shown for clarity. As
illustrated, the tuner 100 contains a low noise amplifier (LNA)
102, having an input connected to the input of the tuner 100. An
input of a mixer 104 is connected to an output of the LNA 102. An
output of the mixer 104 is connected to an input of a first
amplifier (AMP1) 106. An output of the first amplifier 106 is, in
turn, connected to an input of a variable attenuator (ATT) 108. An
input of a second amplifier (AMP2) 110 is connected to an output of
the variable attenuator 108. An output of the second amplifier 110
is connected to an input of an automatic gain controller (AGC) 112,
whose output is connected to another input of the variable
attenuator 108. The output of the second amplifier 110 is also
supplied as an output of the tuner 100.
[0016] A signal of a predetermined frequency range (e.g. from 50
MHz to 860 MHz) is supplied to the LNA 102 through the input of the
tuner 100. The signal is linearly amplified by the LNA 102 before
being passed to the mixer 104. The mixer 104 down-converts the
linearly amplified signal to baseband (0 Hz) or near baseband
before supplying the down-converted signal to the first amplifier
106. The first amplifier 106 amplifies the down-converted signal by
a predetermined amount and then supplies the amplified signal to
the variable attenuator 108. The variable attenuator 108 attenuates
the amplified signal by an amount determined by the control voltage
supplied from the AGC 112. The attenuated signal is then amplified
by another predetermined amount in the second amplifier 110. In the
AGC loop, the AGC 112 receives a feedback signal of the amplified
signal from the second amplifier 110 and uses the feedback signal
to adjust the control voltage, and thus, the amount of attenuation.
For example, if the output of the tuner 100 is within a
predetermined voltage from the maximum output voltage, the AGC 112
controls the variable attenuator 108 to increase the amount of
attenuation while if the output of the tuner 100 is within a
predetermined voltage from the minimum output voltage, the AGC 112
controls the variable attenuator 108 to decrease the amount of
attenuation. The minimum and maximum output voltages may be
determined by the module to which the attenuated output signal is
supplied or some other criterion determined by the system of which
the variable attenuator is a part.
[0017] FIG. 2 illustrates one example of the variable attenuator
108 in FIG. 1. FIG. 3 is an enlargement showing input section 210
of the variable attenuator 200 shown in FIG. 2. As shown, the
variable attenuator 210 contains a pair of first elements (shown
and hereinafter described as resistors R1, R2) and a single second
element (shown and hereinafter described as MOSFET M). The
resistors R1, R2 are connected in series between the input IN and
the output OUT of the variable attenuator 210. The shunt MOSFET M
is connected between a node between the resistors R1, R2 and ground
Vss. The substrate of the MOSFET M is grounded. A control voltage
is connected to the control terminal (gate) of the MOSFET M. The
MOSFET M acts as a shunt resistor, having a large resistance when
off (i.e. when the channel between the source and drain regions is
closed) and having a substantially smaller resistance when on (i.e.
when the channel is open). For example, when the MOSFET M is off,
the resistance may be 10R1, while when the MOSFET M is on, the
resistance may be 0.1R1. Thus, when the MOSFET M is off, the
voltage at the node between the resistors R1, R2 is maximized and
when the MOSFET M is on, the voltage at the node is minimized.
Accordingly, the attenuation varies from a relatively small amount
when the MOSFET M is off to a relatively large value when the
MOSFET M is on.
[0018] However, while the control voltage is able to vary the
attenuation of the variable attenuator of FIG. 3, the slope of the
attenuation curve is linear over only a small range. As the MOSFET
turns on, the slope of the attenuation curve starts to increase. As
the control voltage increases further, this slope reaches a
maximum. Unfortunately, the slope does not stay constant as the
control voltage keeps increasing. At a certain control voltage, the
slope magnitude of the attenuation versus the control voltage curve
begins to decrease. This can be seen as the drain-to-source
resistance Rds of the MOSFET is given by (1).
R ds = L .mu. C ox W ( V gs - V T ) ( 1 ) ##EQU00001##
[0019] In this equation, L is the length of the channel, .mu. is
the mobility of the carriers, C.sub.ox is the capacitance formed
across the oxide, W is the channel width, Vgs is the gate-to-source
voltage, and Vt is the threshold voltage. Equation (1) shows that
the resistance decreases as Vgs increases. The attenuation of the
input signal (in dB) for the circuit shown in FIG. 2 is given by
(2).
Atten ( dB ) = 20 log ( R ds R ds + R ) ( 2 ) ##EQU00002##
[0020] Where R is R1 in FIG. 3. This equation can be simplified and
rewritten as shown by (3).
Atten ( dB ) = 20 log ( 1 1 + RK ( V gs - V T ) ) where K = .mu. C
ox W L ( 3 ) ##EQU00003##
[0021] If (3) is differentiated with respect to Vgs, the result is
shown in (4).
.differential. Atten ( dB ) .differential. V gs = 20 RK log 1 + RK
( V gs - V T ) ( 4 ) ##EQU00004##
[0022] If the source is grounded and the gate is supplied with the
control voltage from the control terminal, as is the case in the
arrangement of FIG. 3, (4) shows that for a control voltage just
above V.sub.T, the slope of the attenuation versus control voltage
curve is approximately -20RKloge. As the control voltage rises, the
slope decreases towards zero. This decrease in attenuation slope
magnitude limits the attenuation range.
[0023] The attenuation range can be increased if the sensitivity is
increased. FIG. 4 is a graph showing the slope of the attenuation
for the embodiment of FIG. 2 and if only input section 210 in FIG.
2 is present in the variable attenuator (i.e. only a single MOSFET
is present). As illustrated in FIG. 4, the slope of the attenuation
for the single MOSFET case is dB linear over only a relatively
small range of voltages, from about 1.3V to about 1.6V.
[0024] For a single MOSFET attenuator, a tradeoff exists between
the attenuation slope and the attenuation range. By increasing the
size (W/L ratio) of the MOSFET, the difference in MOSFET resistance
when the MOSFET is on and when the MOSFET is off increases.
Accordingly, the input signal can be attenuated many orders of
magnitude between the on and off states of the MOSFET. As a result,
the sensitivity of the attenuation control correspondingly
increases. In other words, for very small changes in the gate
voltage, there will be a large change in the attenuation of the
input signal. On the other hand, by decreasing the size of the
MOSFET, the difference in MOSFET resistance when the MOSFET is on
and when the MOSFET is off decreases. Accordingly, the input signal
can be attenuated by a comparatively smaller amount. This
correspondingly decreases the sensitivity of the attenuation
control so that for large changes in the gate voltage, only a
relatively small change in the attenuation of the input signal
occurs.
[0025] Thus, if only a single MOSFET is used as arranged in FIG. 3,
a fixed attenuation range is obtained given a particular
attenuation slope. One limitation as to the applications in which
the variable attenuator may be used is the linearity of the
attenuation curve in dB/V. The bandwidth of the AGC loop can be
increased with increasing linearity of the attenuation slope. As
discussed above, this may be of more importance in mobile
applications, where the received signal strength can vary quickly
over time. However, the linearity of the variable attenuator is
also degraded by the presence of the shunt MOSFET. When the control
voltage is near the threshold voltage of the MOSFET, the MOSFET is
in the saturation region (in which V.sub.gs>V.sub.t and
V.sub.ds>V.sub.gs-V.sub.t, where V.sub.ds is the drain-to-source
voltage) and exhibits non-linear behavior. As the voltage
increases, the gate-to-source voltage of the MOSFET increases until
the MOSFET enters the linear region (in which V.sub.gs>V.sub.t
and V.sub.ds<V.sub.gs-V.sub.t). If an increased attenuation
range is desired, a device of increased size is used. However, as
the MOSFET size increases, the linearity of the variable attenuator
decreases when the control voltage is near the threshold
voltage.
[0026] In more detail, FIG. 2 illustrates one embodiment of the
variable attenuator. As shown, the variable attenuator 200 contains
a pair of series resistors R1 and R2 connected between the input IN
and the output OUT of the variable attenuator 200. A resistor chain
R3, R4, R5, R6 is connected between the control voltage and ground
Vss. The resistors R3, R4, R5, R6 act as voltage dividers between
the control voltage and ground Vss. The nodes between the adjacent
resistors in the resistor chain R3, R4, R5, R6 are connected to
control terminals of the devices M1, M2, M3. Thus, each node
between adjacent resistors R3, R4, R5, R6 provides a predetermined
voltage that is dependent on the control voltage (more
specifically, a ratio of the difference between the control voltage
and Vss) to the terminals of the devices M1, M2, M3. The resistors
in the resistor chain R3, R4, R5, R6, similar to the resistors R1,
R2 between the input IN and the output OUT, may have any desired
resistance. For example, the resistance of resistor R2 may be
0.
[0027] As described above, the MOSFETs M1, M2, M3 act as shunts for
signals passing between the input IN and the output OUT of the
variable attenuator 200, with the gates of the MOSFETs M1, M2, M3
connected to the nodes between the adjacent resistors in the
resistor chain R3, R4, R5, R6. Either n-channel or p-channel
MOSFETs may be used, although n-channel MOSFETs may be more
desirable at least as a smaller device can be used to achieve the
same drain-to-source resistance. The MOSFETs M1, M2, M3 are
connected in parallel between the series resistors R1, R2.
[0028] The MOSFETs M1, M2, M3 are controlled by the voltage at the
associated nodes of the resistor chain R3, R4, R5, R6. This voltage
may be a continuous analog voltage or a discrete digital voltage.
As described above, as the control voltage increases, the
drain-to-source resistance of each MOSFET decreases from a
relatively high resistance when the MOSFET is off continuously
until it reaches a predetermined low resistance when the MOSFET is
on. Thus, the input signal is attenuated by a voltage divider
created by the series resistor pair R1, R2 and the MOSFETs M1, M2,
M3. As the gates of the MOSFETs M1, M2, M3 are supplied with
different voltages, which are all dependent on the control voltage,
the MOSFETs M1, M2, M3 turn on at different times. In other words,
the MOSFETs M1, M2, M3 turn on at different voltage levels of the
control voltage.
[0029] In one embodiment, the MOSFETs M1, M2, M3 have different
sizes. In some embodiments, although each of the channel lengths is
the same, each of the channel widths of the MOSFETs M1, M2, M3 is
W, 8W, and 32 W, respectively. By varying the sizes of the MOSFETs
M1, M2, M3 and turning on the MOSFETs M1, M2, M3 at different
voltage levels of the control voltage, a better tradeoff between
attenuation range and attenuation slope can be obtained. As shown
in FIG. 2, the larger the MOSFET, i.e. the larger the width, the
smaller the voltage applied to the control terminal and thus the
larger the control voltage to turn on the MOSFET. This arrangement
permits the MOSFETs M1, M2, M3 to compensate for non-linearity.
More specifically, the smaller MOSFETs remain in the linear region
when the larger MOSFETs are in the saturation region. As the larger
MOSFETs have smaller resistances than the smaller MOSFETs, and
since the source/drain of the MOSFETs are connected in parallel,
the effect of the saturation of the larger MOSFETs on the
attenuation curve is decreased in significance by the smaller
MOSFETs. This accordingly permits the attenuation to increase
substantially linearly at higher control voltage levels. Thus, as
the control voltage is related to the voltage from the tuner, as
the voltage from the tuner increases, the amount of attenuation
increases.
[0030] The operation of the variable attenuator 200 will be
described with reference to the control voltage verses attenuation
curve of FIG. 4. When operating, an input signal is supplied to the
input IN. If the control voltage is 0 volts, the output voltage
Vout is equal to the input voltage Vin. As the control voltage
rises, MOSFET M1 will turn on first. The slope of the attenuation
versus control voltage curve starts to increase to a maximum value
determined essentially by the voltage divider of resistor R1 and
the resistance of MOSFET M1. Eventually, the slope magnitude starts
to decrease, as shown in equation (4), at which point MOSFET M2
starts to turn on. Thus, the attenuation slope stays substantially
constant until the slope magnitude of the attenuation due to MOSFET
M2 starts to decrease. At this point, it is desired to have MOSFET
M3 start to turn on, etc.
[0031] The slope of the plot of the three MOSFET attenuator is
substantially constant over a much broader range of control
voltages compared to the single MOSFET variable attenuator. With
judicious choice of the MOSFET size and resistor selection for both
the resistor chain and the resistor R1 connected to the input of
the variable attenuator, a particular constant attenuation slope
can be achieved for many orders of magnitude of the attenuation.
Thus, a desired attenuation range and sensitivity to the control
voltage can be achieved simultaneously. The control voltage extends
to a maximum of 3.3V. In the plot, a three MOSFET variable
attenuator with different MOSFET sizes (M3>M2>M1) was
designed to give the same attenuation range as a single MOSFET
variable attenuator. The three MOSFET variable attenuator was also
designed to have an attenuation curve slope of -14 dB/V. As can be
seen, the single MOSFET variable attenuator is only dB/V linear for
a small range of the control voltage, from about 1.3V to about
1.6V. The three MOSFET variable attenuator is dB/V linear with an
approximate slope of -14 dB/V for a much larger control voltage
range, from about 1.3V to about 3.3V. This more linear in dB/V
attenuation slope allows for a larger bandwidth AGC loop. This
enables the variable attenuator to be used in a mobile application
where the received signal strength can vary quickly over time.
[0032] In addition, the intermodulation (IM) distortion products
produced by the multiple MOSFET variable attenuator is
significantly less than that of the single MOSFET variable
attenuator. Since the first device to turn on has the smallest
width (and thus has the smallest resistance), it does not have as
negative of an effect on linearity as a larger device when the
gate-to-source voltage is near threshold. As a larger MOSFET starts
to turn on, the smaller MOSFET(s) is well into the linear region.
Thus, the initial non-linearities associated with the
drain-to-source resistance of the larger MOSFET is decreased due to
the presence of the more linear drain-to-source resistance of the
smaller MOSFET. In the plot of FIG. 4, the linearities of the
variable attenuators were simulated and compared. For a constant
output power level of -25 dBm, the linearity of the variable
attenuators were observed at attenuation levels from -1 dB to -25
dB with a -1 dB step. The worst intermodulation distortion over the
entire attenuation range for a constant output power of -25 dBm was
-56.2 dB for the single MOSFET variable attenuator and -69.25 dB
for the three MOSFET variable attenuator. Thus, the three MOSFET
variable attenuator has an improved intermodulation distortion
level of +13 dB over the single MOSFET variable attenuator as well
as a more constant attenuation slope over the same attenuation
range.
[0033] FIG. 5 illustrates another embodiment of the three MOSFET
variable attenuator. As illustrated, the variable attenuator 500 is
a differential variable attenuator containing a pair of series
resistors R1 and R2 connected between each of the inputs INP, INM
and the respective outputs OUTP, OUTM. A resistor chain R3, R4, R5,
R6 is connected between the control voltage and ground Vss. The
nodes between the adjacent resistors in the resistor chain R3, R4,
R5, R6 are connected to gates of the respective MOSFETs M1, M2, M3.
As before, the MOSFETs M1, M2, M3 have a channel width of W, 8W,
and 32W, respectively. To balance the output signals, a pair of
bias resistors RBIAS are connected in series with each other. The
bias resistors RBIAS connected in parallel with the MOSFETs M1, M2,
M3 are connected between the series resistors R1, R2. The node
between the bias resistors RBIAS is connected to ground Vss.
[0034] Although resistors have been described as the first element
in FIG. 2, other devices may be used, as shown in the embodiment
FIG. 6. The embodiment of FIG. 6 is identical to that of FIG. 2,
except that the series resistors R1, R2 between the input IN and
the output OUT of the variable attenuator have been replaced by
MOSFETs M4, M5. The gates of the series MOSFETs M4, M5 are
connected to a voltage Vc, and thus have a resistance dependent on
the voltage Vc. The series MOSFETs M4, M5 have the same or
different sizes and may have the same or different voltages applied
to their gates. The voltage Vc may be constant or continuously or
discretely variable.
[0035] In other embodiments, any FET or other device(s) can be
used, as long as it provides the desired attenuation
characteristics. For example, MISFETs or variable resistors may be
used rather than MOSFETs. By using CMOS, however, the current drain
of the variable attenuator does not change substantially across the
attenuation range. Similarly, although embodiments using three
MOSFETs are shown, two or more MOSFETs can be used in different
embodiments. Moreover, although the MOSFETs are described as having
different sizes, and thus different drain-to-source resistances,
two or more of the MOSFETs can have the same size while being
connected to different nodes of the resistor chain or multiple
MOSFETs can be connected to the same node. Thus, for example, one
or more of the MOSFETs shown in the figures can be implemented by
multiple MOSFETs, as shown in the variable attenuator 700 shown in
FIG. 7. In one embodiment, the multiple MOSFETs have the same
channel width to provide an effective channel width. In another
embodiment, at least one of the multiple MOSFETs has a different
channel width than another of the multiple MOSFETs.
[0036] Similarly, MOSFETs of various lengths can be used. However,
as the length of the MOSFET increases, so does the minimum
resistance. In addition, any number of resistors can be used. Other
elements such as resistors may be connected in series with the
MOSFETs, for example. A resistor that is much larger than the
source-to-drain resistance may be disposed between the source and
drain of the MOSFET. The variable attenuator may be used in devices
other than tuners, for instance, cable modems, radios, cellular
telephones, PDAs, laptop computers or other communication devices
or systems that use a variable attenuator.
[0037] Accordingly, the specification and figures are to be
regarded in an illustrative rather than a restrictive sense, and
all such modifications are intended to be included within the scope
of present invention. As used herein, the terms "comprises,"
"comprising," or any other variation thereof, are intended to cover
a non-exclusive inclusion, such that a process, method, article, or
apparatus that comprises a list of elements does not include only
those elements but may include other elements not expressly listed
or inherent to such process, method, article, or apparatus.
[0038] It is therefore intended that the foregoing detailed
description be regarded as illustrative rather than limiting, and
that it be understood that it is the following claims, including
all equivalents, that are intended to define the spirit and scope
of this invention. Nor is anything in the foregoing description
intended to disavow scope of the invention as claimed or any
equivalents thereof.
* * * * *